This invention relates to coked coal and more particularly to the continuous production of coke from any type of coal which can be made to coalesce on heating.
In order to satisfy the ever increasing demand for products made from iron since the beginning of the Iron Age, man has had to seek new sources of iron bearing minerals. However, in order to extract the iron content of these minerals, the ore must be smelted with a reducing agent that will react with the elements that are naturally combined with iron and which maintain the iron in its primordial rock status. Traditionally, this reducing agent has been carbon in its crude or purer form. For centuries the carbon source was wood char--"char coal"--autogenously produced by ignition of the outside of a wood pile where the heat so generated penetrates inward to the exclusion of air and carbonizes the raw wood to a black char which contains in excess of 75% carbon.
However, in order to satisfy the demand caused by the exponential increase in population and the corresponding increase in human demand for iron products, the production of wood char-coal reductant was expanded by using externally heated ovens wherein the wood was charred by heat through the oven walls. The heat was created by burning tree bark and branches in order to make maximum use of the tree wood, both as fuel and as feedstock for the char-coal.
As the demand for iron products continued to increase, the availability of trees to be carbonized to the reducing "wood char" needed by the smelter became inadequate. Char-coal production then evolved into a method utilizing "coke ovens".
The first "coke ovens" were of the "traditional" bee-hive design wherein coal was charged to a bee-hive-shaped ceramic structure that could be heated externally by the combustion of coal beneath this ceramic crucible and driving off the hydrogen and oxygen contained in the coal to produce a residue with less than 1% of the matter, contained in the coal, that would crack and distill at temperatures in excess of 2000.degree. F. (1100.degree. C.).
Because of the waste of heat in terms of coal substance lost (from 15% to 50%) a technology was developed that recovered the substances driven off by the pyrolysis. This development was called the "by product" or "slot-type" oven and has dominated the production of a carbon reductant and heat source from coal for the iron and steel industry to the practical exclusion of any other technique.
A reductant made from coal that is designed for use with a modern, high-wind velocity, blast furnace must meet very definite physical and chemical specifications. On the chemical side of the specifications, solid reductants destined for blast furnace use are specified to contain less than 3 wt.% volatile matter on a dry basis when tested via the ASTM method D-271-70.
In addition are implied specifications which require that the volatile matter contained in any reductant used in a blast furnace--regardless of quantity--contain no substance that will produce a "tar" on condensation. While the ash and sulphur content of the reductant are vital from the standpoint of efficiency and statutory requirements, these properties are controlled by the amount of these elements permitted in the raw coal feed. The main chemical control of iron ore reductants from coal remains the volatile content of the "coke" product.
On the physical side of the specifications for such reductants are the strength characteristics of resistance to destruction by abrasion and the resistance to destruction by sudden impact. There are a number of ASTM tests used to measure those qualities. But the most important of those qualities is the ability of the reductant (coke) to withstand the tumbling and dropping action without breaking down into pieces of less that 1/4 inch (6.4 mm) in volume dimensions. To the extent that such break-ups occur, to that same extent, on some relative basis, will the productive capacity of any given furnace be reduced. This quality can only be inferred from the aforementioned tests. Actual use in, and observation of, a given furnace response to a specific solid reductant is the final test.
In order to produce the chemical and physical qualities required, the formulation of a charge to a by-product or bee-hive oven must be controlled. A minimum of about 25% of certain low volatiles matter coals--15% to 25% volatile matter content--must be maintained. "Blending" coals must also have properties that will enhance coke quality. Originally, the United States supply of the low volatile metallurgical coal (although essentially limited to Pennsylvania, West Virginia, Ohio and Alabama) seemed inexhaustible. However, since World War II the demand for iron products has increased not only exponentially to match population growth, but as well by the back log requirements of developing nations and the increase in the material quality of life styles in Western Culture. This explosion in the use of iron has caused a continuing serious depletion in the supplies of metallurgical quality coals, i.e. those coals that can be used in by-products ovens to produce coke which will meet blast furnace specifications as well as fuel for foundry operations.
Many attempts have been made to develop a method that would use almost all the world coal supplies to produce solid metallurgical reductant. A few have been experimentally successful, but only one is of commercial status at the present time. All of these processes included reduction of the coal to sizes less than 3/16 of an inch (4.76 mm) and recombination of those particles, after partial or complete pyrolytic devolatilization, by forming with binder followed by a subsequent pyrolytic devolatilization to reduce the volatile matter and/or eliminate the tars that may form in high temperature applications. No process has successfully used, to advantage, one fundamental property of all coal--the development of an adhesive state in the temperature history during pyrolytic devolatilization.
The most nearly commercial operation grinds the delivered coal to minus 3/16" (4.76 mm), devolatilizes this ground coal via fluidized bed pyrolysis techniques, produces a tar and a char, devolatilized to less than 3% as measured by ASTM method D-271-70, combines the tar and char in a briquetting operation to any desired size and subsequently devolatilizes the final shape in two temperature stages--a low temperature stage using air and a high temperature stage using flue gas--to less than a 3% volatile matter, i.e. strong coke for use in metallurgical processes. This process does not require a specific rank of coal.
Another known process grinds coal to a size amenable to charring in particulate form; the devolatilized char, produced in a recycling transport reactor, yields tar and a carbonization aqueous liquor. The char is heated at or near the softening point of raw coal which is combined with the char and tar in a briquetting operation whereby the heat from the char and the heat developed by pressure briquetting act to soften the scarce coking or binder coal into the raw shapes; these resulting briquettes are devolatilized in a slow heating cycle, in the presence of some air, to less than a 3% volatile matter product. This process requires between 20% and 30% of a high quality metallurgical coal as the binder coal.
Yet another known process, essentially the same as that described immediately above uses hot pelletizing instead of briquetting. This process essentially requires a coal blend similar to the blends fed to "by-product" ovens. The advantage of this process lies in its continuous operation as opposed to the batch sequence used in the "by-product" oven.
Finally, still in pilot stage, is a process wherein dried coal is heated to about 400.degree. F. (204.degree. C.) blended with a tar recovered from devolatilization and formed into a desired shape; the shape is devolatilized by heating with an oxygen-free flue gas which flows upward through a downward flow of solid shapes. At some point, air is introduced to the shaft in order to combust some of the coal and coal gasses to supply heat for devolatilization.
The process of the present invention, described in more detail below, differs from the foregoing processes in that it makes use of the adhesive properties of the coal substance by a carefully controlled heating, in the absence of air, of the shapes which have been formed by mixing ground, raw coal (5% water maximum) with binder inherent or foreign to the process. This heating regimen is carefully matched to the devolatilization rate of the coal with respect to temperature. By doing so, instead of permitting the shapes to melt and/or agglomerate to a weak useless mass, the single process involving melting and agglomeration gives the final product the necessary strength and only requires commonly available, safe equipment.
The process of this invention can utilize any coal that distills off tar-forming substances at any point in the coal's devolatilizing history. The process of this invention differs from the first mentioned prior known process in that no devolatilization prior to forming is used and devolatilization continues uninterruptedly to the finished product. The process of this invention differs from the second and third aforementioned processes in that forming does avoid use of a charred product and of all of the attendant dangers of exposing to oxygen the coal product solids that are at or above the autoignition temperature of the solid product. The process of this invention also has the advantage that it does not require more than one coal feed, although a blend of a multiple of feeds can be used.
The process of this invention differs from last abovementioned processes in that at no point in the heating cycle is air introduced into the system. Accordingly, the dangers that accompany the introduction of oxygen to a combustible material above the point of its autoignition and/or explosive ignition are eliminated. Also, the process of this invention, unlike the earlier process, does not require any special blend of coals.